Dynamic detection of blocking artifacts

ABSTRACT

The present invention relates to a method of processing a sequence of digital images, intended to detect, in a dynamical manner, a grid comprising blocking artefacts. Said method comprises the steps of, a) detecting ( 100 ) a current spatial grid (SG(t)) within a portion of the image constituted by a current field (FLD(t)), b) determining ( 200 ) a current reference grid (RG(t)) from a current spatial grid (SG(t)) and a preceding reference grid (RG(t- 1 )) supplied by a memory MEM ( 150 ), the current reference grid (RG(t)) being subsequently stored temporarily in the memory MEM ( 150 ), and c) correcting COR ( 300 ) blocking artefacts which are present in the current field (FLD(t)) from the preceding reference grid (RG(t- 1 )) so as to supply a processed field (PPP(t)).

The invention relates to a method of processing a sequence of digitalimages, intended to detect a grid corresponding to blocking artefacts,said method comprising a step of detecting a spatial grid within aportion of the image.

The invention also relates to a television receiver comprising aprocessing device using the method of processing images according to theinvention.

The invention notably finds its application in the field of detectingblocking artefacts within a sequence of digital images which haspreviously been encoded and then decoded in accordance with ablock-based encoding technique, for example, the MPEG standard (“MotionPictures Expert Group”) and in the correction of data comprised in theseblocks in order to attenuate the visual artefacts caused by theblock-based encoding technique.

The blocking artefacts constitute a crucial problem for the block-basedencoding techniques using a discrete transform of the discrete cosinetransform DCT type. They appear in the form of block mosaics which aresometimes extremely visible in the decoded image sequences. Theseartefacts are due to a strong quantization subsequent to the discretetransform, which strong quantization causes strong discontinuities toappear at the borders of the encoding blocks.

International patent application WO 01/20912 (docket: PHF99579)describes a method with which a grid corresponding to blocking artefactswithin a decoded digital image can be detected and localized. Thismethod authorizes the detection of three periodical grid sizes of 8×8,10×8 and 12×8 pixels, which grid sizes result from principal formats ofimages used for broadcasting televised digital programs. The 8×8 sizecorresponds to an image sequence encoded in a format of 576 rows of 720pixels, the 10-11-11×8 size corresponds to an encoding in a 576×540format, referred to as encoding format 3/4, and the size 12×8corresponds to an encoding in a 576×480 format, referred to as encodingformat 2/3. The size of the grid is obtained by searching the mostfrequent horizontal and vertical distances between the blockingartefacts. The horizontal and vertical offsets of the grid size withrespect to the origin (0,0) of the image are obtained by searching,among all possible offsets, those which correspond to the presence ofthe largest number of blocking artefacts.

The prior-art method is subsequently based on the redundancy of theperiodical grid size and its offset with respect to the origin of theimage, for successive images. Such a method of processing imagesvalidates a new periodical grid size (and/or offset) if it has beendetected at least a predetermined number of consecutive times.

It is an object of the present invention to propose a data processingmethod which is more efficient.

Indeed, the time parameter is used as a simple validation tool for theprior-art method of processing images, to a certain extent playing therole of a switch with which a grid having given size and offsetparameters can be switched to another completely different grid havingdifferent parameter values.

Moreover, it searches only one grid size and one grid offset withrespect to the origin of the image. But the grid may be distorted withinthe image because of a resampling of the image. This distortion maysometimes be known in advance, as in the case of the 3/4 encodingformat, where the width of the grid varies in accordance with the10-11-11 pattern. However, this variation is mostly arbitrary because itoriginates, for example, from a rate transcoding, an image formatconversion in a 16/9 television receiver, from a 4/3 format to, forexample, a 16/9 format, a zoom in a portion of the image, an A/Dconversion, or even a combination of these different conversions. Inthis case, the prior-art method retains the grid having the mostfrequent size and offset and applies a step of correcting blockingartefacts based on this grid, with a risk of a partial or eveninefficient correction if the grid has a variable size.

To this end, the image processing method according to the invention ischaracterized in that it comprises a step of determining a currentreference grid from a current spatial grid and a preceding referencegrid.

In this way, the image processing method constructs a current referencegrid which is suitable for integrating the modifications of the currentspatial grid with respect to the preceding reference grid, thusreflecting the fluctuations of grid contents as a function of time. Theefficiency of the image processing method is thereby enhanced.

In a particularly advantageous embodiment, a grid comprises sets of atleast one blocking artefact, and the reference grid comprises anindicator associated with a set of at least one blocking artefact, theindicator of the current reference grid being updated from the indicatorof the preceding reference grid, and from the absence or presence of theset of at least one block artefact associated with the indicator in thecurrent spatial grid.

Taking the spatial redundancies of a set of blocking artefacts intoaccount (a set of blocking artefacts being a part of the grid takenindependently and being equal to a block artefact, to a segment ofblocking artefacts or to a row comprising blocking artefacts) by way ofan indicator, and not by the redundancy of the entire grid as in theprior art, allows a more precise and more reliable way of detectingblocking artefacts. The reference grid may thus be modified by adding orsubtracting sets of blocking artefacts in accordance with the value ofthe indicators associated with them. Moreover, the updating of thereference grid renders parallelism possible between the detection of thegrid on the basis of a current image and the correction of the currentimage on the basis of the preceding reference grid.

These and other aspects of the invention are apparent from and will beelucidated, by way of non-limitative example, with reference to theembodiment(s) described hereinafter.

In the drawings:

FIG. 1 is a diagram showing a method of processing images according tothe invention,

FIG. 2 illustrates two artefact profiles p1 and p2 which are principallyencountered in images encoded in accordance with a block-based encodingtechnique, which profiles are represented in the spatial domain and thefrequency domain,

FIG. 3 a illustrates the updating of a reference grid from a currentspatial grid and FIG. 3 b illustrates the comparison between a precedingreference grid and a current spatial grid,

FIG. 4 describes a method of correcting blocking artefacts, and

FIG. 5 describes the principle of correcting a blocking artefact of thetype p2.

The present invention relates to a method of processing a sequence ofdigital images encoded and decoded in accordance with a block-basedencoding technique. In our example, the encoding technique used is theMPEG standard based on the discrete cosine transform DCT, but mayalternatively be any other equivalent standard, such as, for example,the H.263 or H.26L standard. The processing method first relates to thedetection of blocking artefacts due to these block-based encodingtechniques and subsequently to the ensuing applications such as, forexample, post-processing techniques.

FIG. 1 shows diagrammatically the method of processing a sequence ofdigital images according to the invention. Said method comprises thesteps of:

-   -   detecting (100) a current spatial grid (SG(t)) within a portion        of the image constituted by a current field FLD(t)),    -   determining (200) a current reference grid (RG(t)) from a        current spatial grid (SG(t)) and a preceding reference grid        (RG(t-1)) supplied by a memory MEM (150), the current reference        grid (RG(t)) being subsequently stored temporarily in the memory        MEM (150), and    -   correcting COR (300) blocking artefacts which are present in the        current field (FLD(t)) from the preceding reference grid        (RG(t-1)) so as to supply a processed field (PPP(t)).

These steps will be described in greater detail in the followingdescription.

The data processing method according to the invention comprises a stepof detecting the grid within an image. This step may be effected inaccordance with different principles such as that described in, forexample, patent application WO 01/20912.

In the preferred embodiment, the detection of the grid within a field iseffected in accordance with the principle described with reference toFIG. 1. This spatial grid detection first comprises a step of high-passfiltering HPF (110) a portion of a digital image. This portion is, forexample, one of two fields of a frame if the image is constituted by twointerlaced frames. In the preferred embodiment, the high-pass filteringstep is a gradient filtering step using the filter hp1=[1, −1, −4, 8,−4, −1, 1]. This filter is applied horizontally and vertically, row byrow, to pixels of luminance Y(m,n) of the current field FLD(t) of adigital image of the sequence, where m and n are integers between 1 andM and between 1 and N, respectively, corresponding to the position ofthe pixel in the field in accordance with a vertical and a horizontalaxis, respectively, (M=288 and N=720 in, for example, the 576×720encoding format). The result of this filtering operation is preferablyconstituted by two cards of discontinuity pixels, a horizontal card Ehand a vertical card Ev comprising filtered coefficients Yfh and Yfv,respectively.

The spatial detection of the grid must be able to distinguish thediscontinuities corresponding to visible blocking artefacts from thosecorresponding to natural contours or non-visible blocking artefacts.

That is why the spatial grid detection comprises a threshold step THR(120) intended to detect natural contours and non-visible artefacts. Tothis end, a coefficient value filtered horizontally Yfh(m,n) and/orvertically Yfv(m,n) must be between two thresholds so as to be able tocorrespond to a block artefact. The first threshold S1 corresponds to avisibility threshold, whereas the second threshold corresponds to thelimit from which the pixel of position (m,n) corresponds to a naturalcontour. The condition is preferably taken for the absolute value ofcoefficients filtered as follows:S1<|Yfh(m,n)|<S2 and S1<|Yfv(m,n)|<S2, with S1=0.5 and S2=20.

The detection of the spatial grid also comprises a step of extractingEXT blocking artefacts (130) suitable for detecting a first type (131)and a second type (132) of block artefact. The selection of pixelscorresponding to blocking artefacts is performed as a function of thevalues of the filtered coefficients Yf corresponding to thediscontinuity pixels. FIG. 2 illustrates the two artefact profiles p1and p2 in the spatial domain as well as their representation in thefrequency domain after filtering with the filter hp1. The first profilep1 corresponds to a standard blocking artifact, while the second profilep2 corresponds to a block artefact which is present in an image that hasbeen subjected to a re-sampling operation or an equivalent processingoperation. In the spatial domain, the first profile p1 is a simple stepof a staircase, while the second profile p2 is a double step of astaircase. In the frequency domain, the first profile p1 is expressed bya peak, while the second profile p2 is expressed by a double peak.

A vertical artefact corresponding to profile p1 is detected by scanningthe vertical card Ev in accordance with a horizontal directioncorresponding to the row m if the following condition is satisfied:|Yfv(m,n)|>|Yfv(m,n+k)| with k=−2, −1, +1, +2.The border of the block is localized between the pixel of position (m,n)and that of position (m,n+1) if |Y(m,n)−Y(m,n−1)|<|Y(m,n)−Y(m,n+1)| andbetween the pixel of position (m,n−1) and that of position (m,n) in theopposite case.

An artefact corresponding to profile p2 is detected if the followingcumulative conditions are satisfied:f1·|Yfv(m,n)|<(|Yfv(m,n−1)|+|Yfv(m,n+1)|)|Yfv(m,n−1)|>f2·|Yfv(m,n−2)||Yfv(m,n+1)|>f2·|Yfv(m,n+2)|with f1=6 and f2=2 in the preferred embodiment.The border of the block is localized between the pixel of position(m,n−1) and that of position (m,n).

The detection of a horizontal artefact corresponding to each profile p1and p2 is effected in a similar manner by scanning the horizontal cardEh comprising the coefficients Yfh(m,n) filtered in a vertical directioncorresponding to the column n.

The detection of the spatial grid also comprises a step of searching GL(140), within the current field, rows of pixels having a high density ofsegments of elementary blocking artefacts as compared with neighboringrows. This search step is performed for the rows comprising blockingartefacts of the first type (141) or blocking artefacts of the secondtype (142), the grid rows thus obtained being re-assembled (143) forforming the current spatial grid SG(t).

To this end, the search step first comprises a selection sub-stepintended to select segments in a horizontal or vertical row of the cardof discontinuity pixels, which segments comprise a number of consecutiveblocking artefacts which is higher than a predetermined threshold S0.Indeed, the isolated discontinuities generally correspond tosupplementary noise, while the blocking artefacts which are due to acoarse quantization of the DCT coefficients generally cause linearfaults to appear along the encoding blocks. The value S0 of thepredetermined threshold must not be too low so as not to favor the falsedetections. It must neither be too high so as not to constrain theselection too much by reducing the number of segments of detectedelementary blocking artefacts. In practice, the value S0 is fixed at 3for a field of 288 rows of 720 pixels.

The search step also comprises a sub-step of computing a level Nb_(i) ofthe blocking artefacts per row L_(i), i being an integer correspondingto the number of the row in the field. In the preferred embodiment, thelevel of the blocking artefacts is obtained by counting the number ofpixels associated with the segments of elementary artefacts present in arow. By way of variant, the level of blocking artefacts may be obtainedby adding the values of the filtered coefficients Yf of thediscontinuity pixels corresponding to the elementary artefacts of theselected segments in a row.

The search step finally comprises a sub-step of determining grid rows, arow being detected as such by comparison with a set of neighboring rows.

In the case of the first profile p1, a row L_(i) is determined as beinga row of the grid based on a comparison of block artefact levels of acurrent row L_(i), of the row which precedes immediately L_(i−1) and ofthe row which follows immediately L_(i+1), if:Nb _(i)>α(Nb _(i−1) +Nb _(i) +Nb _(i+1)) and Nb _(i) >T1·Nbwhere α is a coefficient which is equal to ⅔ in our example for thedetection of vertical rows, and to 3/5 for the detection of horizontalrows; T1 is a minimum percentage of artefacts in a row with which thisrow can be considered to belong to the grid, which percentage is takento be equal to 20% in our example, and wherein Nb is the number ofpixels per row, i.e. 720 or 288 in our example.

In the case of the second profile p2, a row L₁ is determined as being arow of the grid based on a comparison of block artefact levels of acurrent row L_(i) and of the rows which precede immediately L_(i−1) andL_(i−2) and follow immediately L_(i+1) and L_(i+2) if:Nb _(i)>β(Nb _(i−2) +Nb _(i−1) +Nb _(i) +Nb _(i+1) +Nb _(i+2)) and Nb_(i) >T2·Nbwherein β is a coefficient which is equal to 2/3 in our example; T2 is aminimum percentage of artefacts in a row, which is equal to 20% in ourexample. The condition Nb_(i)>T2·Nb provides the possibility ofcontrolling the reliability of the system; by increasing the value ofT2, the risk of false detections is reduced.

The detection of a spatial grid, which will now be described, issuitable for detecting a current spatial grid SG(t) for a current fieldFLD(t). The method of processing images according to the inventioncomprises a step of determining (200) a reference grid RF(t) fromparameters of the current spatial grid SG(t) and a preceding referencegrid RG(t-1). These parameters are, for example, the number of rows ofthe grid or the value of a confidence indicator associated with a gridrow, as we will see hereinafter.

The detection of the reference grid is shown diagrammatically in FIG. 1and comprises three principal steps.

First, it comprises a step of selecting (210) a mode of operation fromstatistics of current spatial grids SG(t) and temporal preceding gridsRG(t-1). In the preferred embodiment, there are 3 modes of operation.The first mode of operation is a mode of initializing INIT (220) thereference grid, the second mode of operation is a mode of modifying MOD(240) the reference grid and the third mode of operation is a mode ofconfirming STAB (250) the reference grid.

The selection step opts for the initialization mode if differentnon-cumulative conditions are satisfied. In accordance with a firstcondition, this mode of operation is activated by an exteriorre-initialization due to, for example, a change of program or a changeof channel, involving a change of the sequence of digital images to beprocessed. In accordance with a second condition, the initializationmode is activated by a strong increase of the number of grid rows in thecurrent spatial grid SG(t) with respect to the number of grid rows inthe preceding spatial grid SG(t-1). In our example, the initializationmode is activated if the number of grid rows of the current spatial gridSG(t) is higher than 3 times the number of grid rows of the precedingspatial grid SG(t-1). In accordance with a third condition, theinitialization mode is activated if a large part of the grid rows of thecurrent spatial grid SG(t) is offset with respect to the grid rows ofthe preceding reference grid RG(t-1). This is the case in our example ifthe number of grid rows of the current spatial grid SG(t) offset withrespect to the grid rows of the preceding reference grid RG(t-1) (i.e.the total number of horizontal and vertical grid rows of the currentspatial grid SG(t) which do not belong to the preceding reference gridRG(t-1)) is higher than one third of the total number of grid rows ofthe preceding reference grid RG(t-1). Finally, the initialization modeis activated if no current spatial grid SG(t) is detected. This isnotably the case when the number of grid rows is lower than apredetermined threshold Smin, as a function of horizontal H and verticalV dimensions of the field and is equal, in our example, to:Smin=(H+V)/48.

The initialization mode (220) consists in reconstructing the currentreference grid RG(t) from the current spatial grid SG(t). It alsoconsists in giving a maximum value, equal to 5 in our example, to aconfidence indicator associated with each grid row. By way of variant,the initialization mode (220) can reconstruct the current reference gridRG(t) from the current spatial grid SG(t) and the preceding spatial gridSG(t-1).

The determination of the reference grid also comprises a step ofcontrolling the stability CTRL (230) following the initialization (220).This control step has the object of detecting instability in thedetection of the reference grid, which instability is notably due toseveral successive re-initializations. This is notably the case if thesequence of processed digital images is an original sequence, i.e. asequence of images which has not been encoded and then decoded. The stepof controlling the stability thus detects a predetermined number ofsuccessive re-initializations, equal to 5 in our example, and generatesan indication with which a step of correcting the current field FLD(t)cannot be performed.

The selection step opts for the modification mode (240) if theinitialization mode has not been selected and if there is a largesimilarity between the current spatial grid SG(t) and the precedingreference grid RG(t-1). This is the case, in our example, when thenumber of grid rows differing between the current spatial grid SG(t) andthe preceding reference grid RG(t-1) (i.e. the total number ofhorizontal and vertical grid rows of the current spatial grid SG(t)which do not belong to the preceding reference grid RG(t-1) plus thetotal number of horizontal and vertical grid rows of the precedingreference grid RG(t-1) which do not belong to the current spatial gridSG(t)) is smaller than one third of the grid rows of the precedingreference grid RG(t-1)).

The modification mode (240) consists in incrementing or decrementing theconfidence indicators associated with the grid rows of the precedingreference grid RG(t-1) in order to obtain the current reference gridRG(t), a confidence indicator being incremented or decremented inaccordance with the presence or absence, respectively, of the grid rowassociated with said indicator in the current spatial grid (SG(t)). Themodification mode also consists in completing the current reference gridRG(t) with respect to the preceding reference grid RG(t-1) with gridrows which are present in the current spatial grid SG(t) and which werenot in the preceding reference grid RG(t-1) or, in contrast, towithdraw, from the current reference grid RG(t) with respect to thepreceding reference grid RG(t-1), the grid rows whose confidenceindicator, once decremented, has become equal to 0.

FIG. 3 a illustrates the updating of a reference grid RG from a currentspatial grid SG(t). Each grid comprises a certain number of grid rows ofthe type p equal to 1 for a grid row comprising blocking artefacts ofthe type p1, shown in grey in FIG. 3, or of the type p equal to 2 for agrid row comprising blocking artefacts of the type p2 shown in black inFIG. 3. After the update, the current reference grid RG(t) hasincremented the confidence indicators of the grid rows which are presentin the preceding reference grid RG(t-1) and in the current spatial gridSG(t), has set to one the confidence indicators of the grid rows whichare solely present in the current spatial grid SG(t), and hasdecremented the confidence indicators of the grid rows which are solelypresent in the preceding reference grid RG(t-1), the value of theconfidence indicators remaining between 0 and 5 in our example. The gridrows, whose confidence indicator value is lower than a predeterminedvalue Sconf equal to 3 in our example, shown in broken lines in FIG. 3a, will not be corrected in the correction step.

FIG. 3 b illustrates the comparison between a row of the preceding gridand of the current spatial grid. The rows in broken lines lengthen thegrid rows of the preceding reference grid RG(t-1). Five grid rows of thecurrent spatial grid SG(t) are not aligned with the grid rows of thepreceding reference grid RG(t-1), i.e. more than one third of the 13grid rows are found in the preceding reference grid RG(t-1). In thiscase, the selection step thus opts for the initialization mode, in whichthe third condition is satisfied.

Finally, the selection step opts for the confirmation mode STAB (250) bydefault when none of the other modes is selected.

The confirmation mode STAB (250) consists in conserving the precedingreference grid: RG(t)=RG(t-1), and in preferably incrementing theconfidence indicators of the grid rows which are higher than or equal tothe predetermined value Sconf, equal to 3 in our example.

The temporal detection of the grid finally comprises a step of refiningREF (260) the distance between the grid rows, which step is acontinuation of the mode of operation which has been selected. Therefining step has for its object to verify whether the grid rows of thecurrent reference grid RG(t), which will be obtained, are within a givenrange of values. Indeed, the space between the grid rows should neitherbe too large nor too small. To this end, the refining step determines anaverage distance from the distances between two successive grid rows,both in accordance with a horizontal direction davgH and a verticaldirection davgV, while the distance between two successive grid rowsmust be between a minimum and a maximum boundary so as to be taken intoaccount. These minimum and maximum boundaries correspond to a minimumand a maximum size of the encoding blocks. In our example, the minimumboundary is 6 in the horizontal direction and 3 in the verticaldirection; the maximum boundary is 21 in any of the two directions.Subsequently, the refining step verifies whether the distance betweentwo horizontal or vertical rows is larger than the distance dh or dv,respectively, such that dh is the maximum value between davgH and 6, anddv is the maximum value between davgV and 3. If a row detected as beinga new grid row in the reference grid RG(t) does not comply with theseconditions, it is withdrawn from the reference grid.

An application of the data processing method according to the inventionis constituted by post-processing images, intended to correct theblocking artefacts which are present in the grid rows. The correctiondepends on the confidence indicator value of a grid row, the correctionbeing applied, as we have seen hereinbefore, when said indicator ishigher than or equal to a predetermined value Sconf which is equal to 3in our example. It depends also on the type p of the grid row.

If the block artefact corresponds to the profile p1, the correctiondescribed with reference to FIG. 4 is applied. The method of correctingblocking artefacts comprises the steps of

-   -   computing a first discrete cosine transform DCT1 (41) of a first        set of N data u, situated at the left or above the border of the        block,    -   computing a second discrete cosine transform DCT1 (42) of a        second set of N data v, situated at the right or below the        border of the block and adjacent to the first set,    -   computing a global discrete cosine transform DCT2 (43) of a set        of 2N data w corresponding to the concatenation CON (40) of the        first and second sets and providing a set of transformed data W,    -   determining PRED (44) a predicted maximum frequency kwpred from        the transformed data U and V obtained from the first (41) and        the second (42) transform DCT1, computed in the following        manner:        kwpred=2.max(kumax, kvmax)+2        with kumax=max(k∈{0, . . . , N−1}/abs(U(k))>T)        kvmax=max(k∈{0, . . . , N−1}/abs(V(k))>T)        -   where T is a threshold which is different from zero,    -   correcting ZER (45) by setting the odd transformed data W from        the global discrete cosine transform to zero, whose frequency is        higher than the predicted maximum frequency, yielding corrected        data W′,    -   computing an inverse discrete cosine transform IDCT2 (46) of the        corrected data, yielding filtered data w′ which are subsequently        intended to be displayed on the screen.

If the blocking artefact corresponds to the profile p2, the correctionmust be modified considerably. Indeed, the position of the border of theblock must be given more precisely because of the double step of thestaircase corresponding to the profile p2, as illustrated in FIG. 5. Tothis end, the correction method preliminarily comprises a step ofre-adjusting the luminance value of the intermediate pixel p(n) intendedto give said luminance value the luminance value of the pixel which issituated directly on its right p(n+1). The previously described stepsare then applied, in which the border of the block is present at theleft of the intermediate pixel, which then forms part of the segment v.By way of variant, it is alternatively possible to choose the luminancevalue of the intermediate pixel to correspond to that of the pixel onthe left, or to that of the pixel having the nearest luminance value. Inboth cases, the positioning of the segments u and v is adaptedaccordingly so as to apply the correction step.

It is possible to implement the processing method according to theinvention by means of a television receiver circuit, said circuit beingsuitably programmed. A computer program stored in a programming memorymay cause the circuit to perform the different operations describedhereinbefore with reference to FIG. 1. The computer program may also beloaded into the programming memory for reading a data carrier such as,for example, a disc comprising said program. The reading operation mayalso be performed by means of a communication network such as, forexample, the Internet. In this case, a service provider will put thecomputer program in the form of a downloadable signal at the disposal ofthose interested.

Any reference sign between parentheses in the present text should not beconstrued as being limitative. Use of the verb “comprise” and itsconjugations does not exclude the presence of elements or steps otherthan those stated in the claims. Use of the article “a” or “an”preceding an element or step does not exclude the presence of aplurality of such elements or steps.

1. A method of processing a sequence of digital images, intended todetect a grid (SG, RG) corresponding to blocking artifacts within saidsequence of digital images using a circuit suitably programmed toperform the steps of: detecting a spatial grid (SG) within a portion ofthe image, determining a current reference grid (RG(t)) from a currentspatial grid (SG(t)) and a preceding reference grid (RG(t-1)), based ona row comparison between said current reference grid and said precedingreference grid, wherein a number of grid rows differing between thecurrent spatial grid SG(t) and the preceding reference grid RG(t-1) issmaller than one third a number of grid rows of the preceding referencegrid RG(t-1); determining corrected blocking artifacts in said currentreference grid (RG(t)) based on artefacts within said current spatialgrid SG(t) and artefacts within said preceding reference grid RG(t-1);assigning said corrected blocking artefacts to said current referencegrid RG(t); and outputting said current reference grid RG(t), whereinsaid grid (SG, RG) comprises sets of at least one block artefact withineach row and, wherein an indicator of the current reference grid (RG(t))is updated from the corresponding indicator of the preceding referencegrid (RG(t-1)) and from a presence or absence of the set of at least oneblock artefact associated with said indicator in the corresponding rowof said current spatial grid (SG(t)), said indicator (ind) beingassociated with a set of at least one block artefact.
 2. An imageprocessing method as claimed in claim 1, wherein the set of blockingartefacts is constituted by a row of the portion of the image having ablocking artefact density which is higher than that of the neighboringrows.
 3. An image processing method as claimed in claim 1, wherein thestep of detecting the spatial grid is intended to perform a high-passfiltering operation on the portion of the image, such that at least onecard of discontinuity pixels is supplied, and to detect a first type(p1) of block artefact and a second type (p2) of block artefact from theat least one card of discontinuity pixels.
 4. An image processing methodas claimed in claim 1, wherein said step of determining correctedblocking artefacts that are present in the current reference grid(RG(t)) is performed in accordance with their type (p1, p2).
 5. An imageprocessing method as claimed in claim 1, wherein the step of determiningcorrected blocking artefacts that are present in a set of blockingartefacts of the current reference grid (RG(t)) is performed inaccordance with a value of the indicator (ind) associated with said set.6. A television receiver comprising: a processing device-to detect areference grid (RG) within a sequence of digital images executing thesteps of: detecting a spatial grid (SG) within a portion of the image,determining a current reference grid (RG(t)) from a current spatial grid(SG(t)) and a preceding reference grid (RG(t-1)), based on a rowcomparison between said current reference grid and said precedingreference grid, wherein a number of grid rows differing between thecurrent spatial grid SG(t) and the preceding reference grid RG(t-1) issmaller than one third a number of grid rows of the preceding referencegrid RG(t-1), wherein said SG and RG grids each comprise sets of atleast one block artefact within each row of said grids and wherein thereference grid (RG) comprises an indicator (ind) associated with a setof at least one block artefact, wherein an indicator of the currentreference grid (RG(t)) is updated from the corresponding indicator ofthe preceding reference grid (RG(t-1)) and from a presence or absence ofthe set of at least one block artefact associated with said indicator inthe corresponding row of said current spatial grid (SG(t)) wherein agrid (SG, RG) comprises sets of at least one block artefact within eachrow and, wherein an indicator of the current reference grid (RG(t)) isupdated from the corresponding indicator of the preceding reference grid(RG(t-1)) and from a presence or absence of the set of at least oneblock artefact associated with said indicator in the corresponding rowof said current spatial grid (SG(t)), said indicator (ind) beingassociated with a set of at least one block artefact; correctingblocking artefacts in accordance with a value of the indicator (ind)associated with each of said sets; assigning said corrected blockingartefacts to said current reference grid RG(t); and displaying saidcurrent reference grid containing said corrected blocking artefacts. 7.A device for processing a sequence of digital images, intended to detecta grid (SG, RG) corresponding to blocking artefacts within said sequenceof digital images, said device comprising: means for detecting a spatialgrid (SG) within a portion of the image, means for determining a currentreference grid (RG(t)) from a current spatial grid (SG(t)) and apreceding reference grid (RG(t-1)), based on a row comparison betweensaid current reference grid and said preceding reference grid, wherein anumber of grid rows differing between the current spatial grid SG(t) andthe preceding reference grid RG(t-1) is smaller than one third a numberof grid rows of the preceding reference grid RG(t-1); means forcorrecting the blocking artefacts which are present in the currentreference grid (RG(t)) in accordance with a value of the indicator (ind)associated with each of said sets wherein said grid (SG, RG) comprisessets of at least one block artefact within each row and, wherein anindicator of the current reference grid (RG(t)) is updated from thecorresponding indicator of the preceding reference grid (RG(t-1)) andfrom a presence or absence of the set of at least one block artefactassociated with said indicator in the corresponding row of said currentspatial grid (SG(t)), said indicator (ind) being associated with a setof at least one block artefact; and means for outputting said correctedblocking artefacts.
 8. A non-transitory computer readable storage mediumcomprising a set of instructions, stored in a programming memory, which,when loaded into a circuit, causes said circuit to perform: detecting aspatial grid (SG) within a portion of the image, determining a currentreference grid (RG(t)) from a current spatial grid (SG(t)) and apreceding reference grid (RG(t-1)), based on a row comparison betweensaid current reference grid and said preceding reference grid, wherein anumber of grid rows differing between the current spatial grid SG(t) andthe preceding reference grid RG(t-1) is smaller than one third a numberof grid rows of the preceding reference grid RG(t-1) wherein said a grid(SG, RG) comprises sets of at least one block artefact within each rowand, wherein an indicator of the current reference grid (RG(t)) isupdated from the corresponding indicator of the preceding reference grid(RG(t-1)) and from a presence or absence of the set of at least oneblock artefact associated with said indicator in the corresponding rowof said current spatial grid (SG(t)), said indicator (ind) beingassociated with a set of at least one block artefact: correctingblocking artefacts based on artefacts within said current spatial gridSG(t) and artefacts within said preceding reference grid RG(t-1);assigning said corrected blocking artefacts to said current referencegrid RG(t); and outputting said corrected blocking artefacts.